Beware of Ransomeware.

It was first reported in England — hackers gained access to the National Health Service computers, effectively shuttering the entire system. Patients were told to stay home; doctors and nurses were unable to access email or medical records and had to take notes by hand. The hackers demanded a ransom, to be paid in bitcoin.

By Friday afternoon, though, it was clear that this was not a limited attack. Businesses in at least 11 other countries reported similar cyberattacks. Many were paralyzed.

There’s still a lot we don’t know. (We’ll be updating this post!) But here’s what we do know, so far:

How, exactly, does this ransomware work?

As its name implies, ransomware works like a hostage-taker.

Once your computer is infected, the attack can do a couple of things. One common approach: Your files will be encrypted or converted into a different language for which only the hacker has the cipher. Often, you won’t even know you’ve been targeted until you try to open a file.

Another, more damaging version is what happened Friday: The ransomware locks you out of your entire system. During the attack in England, computer screens showed a message demanding $300 in bitcoin in exchange for the decryption key that would unlock the files. Victims had three days to pay before the fee was doubled. (Something very similar happened to a hospital system in Los Angeles a couple of months ago. The hospital ended up paying about $17,000. The hackers even set up a help line to answer questions about paying the ransom.)

Source: NHS cyber attack: What it looks like and what you need to know – The Washington Post